Automated circuit board testers are testers linked to a transport system that picks circuit boards from a stack, supplies them to a testing device, and from there to a restacker for restacking the circuit boards into one or more stacks. Accordingly, such automated circuit board testers comprise an unstacker, a testing device, and a restacker.
In the unstacker, the circuit boards, supplied in a stack, are picked one after the other and transported to the testing device. In the testing device the circuit boards are tested by device of contact and short-circuit tests known as such. From the testing device, the tested circuit boards are moved on to the restacker where they are preferably deposited in two separate chutes distinguishing faulty circuit boards from faultless circuit boards.
From German Patent Document DE 34 42 123 A1 an unstacker for picking circuit boards one after the other from a stack is known. This unstacker features a chute in an inclined arrangement open at the top, in which a stack of circuit boards to be unstacked is located. At the bottom end of the chute, a roller is provided parallel to a side edge of the lowermost board. This roller has a step running parallel to the side edge of the lowermost board to grip the side edge and extract the lowermost board from the stack of boards and to place it by means of an ejector lever onto a board placement table.
In the case of this unstacker, the chute can be continually replenished without having to interrupt continual operation. The drawback in this arrangement is, however, that the complete weight or load of the stack is borne by the lowermost board, as a result of which stacking height is restricted since the lowermost board must not be exposed to excessive loading. The circuit boards need to have a thickness permitting clamping of the circuit boards not to be picked. The outer contour of the circuit boards must comprise at least two straight parallel edges to be precisely gripped by the roller. In addition to this, and especially in the case of electric circuit boards, there is a risk of the circuit boards being damaged during picking, due to the lowermost board being shifted out from under the stack of circuit boards arranged above.
To obviate these drawbacks unstackers exist which remove in each case the topmost circuit board from a stack of circuit boards by a suction device and place it on a transport belt. This kind of unstacking is termed "pick and place".
In this arrangement the topmost circuit board is totally free of load and picking is done practically independently of the outer contour. The stack of circuit boards is located in a vertical chute in which the stack is shifted upwards by means of a pusher or the like. The drawback in this unstacker device, however, is that to replenish the chute all operations need to be halted to introduce a new stack into the chute, and the pusher has to be retracted to its starting position.